Received February 19, 2008; Revision received July 29, 2008
Alkylation of a cysteine residue in papain with a pyridoxamine (PX)
cofactor was carried out. The resulting semisynthetic enzyme
(papain–PX) has no detectable protease activity but has the
ability to catalyze enantioselective reductive amination of
α-keto acids. The conjugate was characterized by ion-exchange
chromatography, and the optimal reaction conditions were found. We
report that papain–PX reductively aminates the alkyl side chain
of functionalized α-keto acids to give the respective
α-amino acids with high enantioselectivities, greater than 70%.
Based on these studies, we propose a new model for the catalytic
activity of the semisynthetic enzyme with Interchem software. The
results of the study demonstrate the effectiveness of the modified
enzyme and its potential for engineering new catalytic specificity.
KEY WORDS: semisynthetic enzyme, papain, pyridoxamine, chemical
modification